1. Field of the Invention
The present invention generally relates to image forming apparatuses having a structure for preventing bubbles from entering a recording head or remaining in a fluid supply channel.
2. Description of the Related Art
There are various types of image forming apparatuses, such as printers, facsimile machines, copy machines, plotters, and multifunction peripherals incorporating multiple image forming functions. For example, an inkjet recording apparatus is a fluid-discharging type printer that discharges droplets of ink using a recording head. The discharged droplets of ink attach onto a recording medium, such as a sheet of paper, an OHP sheet, or any other material onto which ink droplets or other fluid can be caused to attach in order to form, print, record, or transfer an image on the recording medium. The image forming apparatus of the fluid-discharging type includes a serial type and a line type. In the serial type, the recording head is moved in a main-scan direction as it discharges ink droplets. In the line type, the recording head discharges ink droplets without moving.
The recording medium on which an image is formed by the image forming apparatus of the fluid-discharge type may include various materials, such as paper, threads, fibers, cloth, leather, metal, plastics, glass, wood, and ceramics. The “image” printed, formed, recorded, or transferred, for example, onto the recording medium may include not only meaningful characters or figures but also random or apparently meaningless shapes or patterns. The “ink” may include a recording fluid, a fixing-treatment fluid, a DNA sample, a resist fluid, or any other fluid capable of forming an image on the recording medium.
A line-type image forming apparatus (such as an inkjet recording apparatus) is known that includes a recording head unit comprising an arrangement of plural heads, and an ink supply system capable of handling deaerated ink and having a main ink tank and a hermetically sealed sub-tank. The sub-tank may be formed of a flexible member (such as a flexible film) and capable of containing ink supplied from the main tank and also supplying the ink to the recording head. The image forming apparatus may further include a distributing member (distributor tank) for supplying (distributing) ink to the respective heads of the recording head unit, and a mechanism for ejecting bubbles collected in the distributor tank via an ink supply channel.
A discharge nozzle of the recording head of such an image forming apparatus is very fine (such as 24 μm in diameter), and oxygen may collect gradually in the nozzle if the ink contains a large amount of dissolved oxygen, possibly causing an ink discharge failure. If there are bubbles of air in the ink in the ink supply channel including the distributing member, bubbles may be supplied to the heads, or a required amount of ink may not be discharged out of the heads due to the high level of dissolved oxygen in the ink, thus failing to obtain desired discharge characteristics. Thus, there is a need for a mechanism for ejecting or removing such bubbles of air from the ink supplied to the distributing member.
Patent Document 1 discloses that a plurality of bubble flow-regulating channels are provided in an ink distributor in order to guide the bubbles to a bubble-collecting space in the distributor, wherein the ink level of the bubbles collected via the bubble flow-regulating channels is detected by float switches. Patent Document 2 discloses that a buffer tank is provided between an ink circulating channel on the printer end and a manifold, so that the ink is once collected in the buffer tank and then supplied to the manifold. Patent Document 3 discloses that a bubble capturing unit is provided to catch bubbles in a fluid supply channel between a main tank and a sub-tank. The bubble catching unit includes a fluid channel portion that is partly made of a flexible material, an actuator, and a valve. The fluid channel portion has a fluid inlet end disposed at a water-head position higher than a fluid outlet end. The bubbles in the fluid that flows through the fluid supply fluid channel are captured and then ejected by opening the valve while the fluid channel portion is contracted by the actuator.
Patent Document 4 discloses a branch member (distributing member) having fluid channels wherein ink in the fluid channels is supplied at the bottom. The branch member has an inclined ceiling and an opening channel at the highest portion of the ceiling where bubbles can be collected and ejected to the atmosphere via the opening channel. Patent Document 5 discloses a bubble separator that includes four separating walls for temporarily blocking the entry of ink via an inlet in order to separate the bubbles from the ink, thus preventing the bubbles from entering the recording head which would cause a discharge failure. Patent Document 6 discloses that a first ink chamber is communicated with an ink cartridge and a second ink chamber is communicated with a recording head, wherein the two chambers are communicated via a thin fluid channel configured to prevent bubbles from entering the second ink chamber.
As mentioned above, if the bubbles in the fluid channels (“common fluid chamber”) in the distributing member for distributing the ink to plural heads enter the heads, a droplet discharge failure (such as the bending of the discharge path or the inability to discharge) may be caused, resulting in failure to perform a stable image forming operation. Particularly, if the common fluid chamber is designed such that the ink flows into the common fluid chamber in the same direction in which the heads are arranged, the bubbles in the ink may be easily carried to the heads via ink supply openings of the heads.
In order to reduce or prevent ripples in the flow of the ink in the common fluid chamber of the distributing member, the wall surfaces of the common fluid chamber may be partly formed of flexible material, and a resilient member (damper unit) for biasing the flexible wall surfaces may be provided in the common fluid chamber. When such a damper unit is installed, it is necessary to prevent an increase in size (such as height) of the distributing member.
Further, as mentioned above, although the distributing member for distributing ink to plural heads may be provided with a bubble-ejecting unit, bubbles may remain in a supply channel for supplying ink to the channels (common fluid chamber) in the distributing member and block the flow of ink to the distributing member, resulting in a failure to sufficiently refill the common fluid chamber.    Patent Document 1: JP2005-096208(A)    Patent Document 2: JP2003-019811(A)    Patent Document 3: JP2008-183746(A)    Patent Document 4: JP2009-012452(A)    Patent Document 5: JP2007-136760(A)    Patent Document 6: JP10-329330(A)